The present invention relates to a novel process for the preparation of 3-menthene and more particularly to a novel selective hydrogenation process utilizing a homogeneous ruthenium catalyst complex therefor.
Fahey reports that cyclic polyenes can be hydrogenated to selectively form cyclic monoenes in the presence of certain ruthenium (II) catalyst complexes (see U.S. Pat. Nos. 3,804,914 and 3,925,494; JOC, Vol. 38, pages 80-87 and 3343-3348, 1973; and Catalysis in Organic Synthesis--1976, Acad. Press, pages 287-304). Of particular interest to Fahey is the selective hydrogenation of 1,5,9-cyclododecatriene for forming cyclododecene.
The present invention is based on the discovery that homogeneous ruthenium (II) catalyst complexes can catalyze the hydrogenation of specific paramenthadienes to selectively form 3-menthene which is valuable intermediate in the synthesis of menthol, for example. Such selective hydrogenation process for the formation of 3-menthene is suprising since it is well known that conventional catalytic hydrogenation of such para-menthadienes using, for example, nickel or palladium catalysts, produces predominantly 1-menthene or 2-menthene which then must be isomerized to 3-menthene.